Technical Field
The invention relates to antibodies, particularly, to those capable of binding to human mIgA-expressing B cells.
Description of Related Art
Human IgA has two subclasses, IgA1 and IgA2, respectively, and the IgA1 subclass is predominant in serum (approximately 80%). IgA1 and IgA2 contain the heavy chains α1 and α2, respectively. Both Most α1 and α2 have a secretory (a) and membrane-bound (ma) forms that are translated from two different sets of mRNA derived from the same RNA transcript, with the mRNA of ma containing a membrane exon. Most serum IgA are monomeric forms (around 90%) rather than dimeric or multimeric forms abundant in secretions.
Secretory IgA (SIgA) is the most abundant immunoglobulin present in external secretions. It serves as the humoral immunity against invading microorganisms at the mucosal surface and maintains the balance of dietary antigens in gastrointestinal ducts. SIgA produced by local plasma cells beneath the epithelium layer is in predominantly dimeric or multimeric forms in association with a J chain. Through interaction with polymeric Ig receptors (pIgR) on the basolateral membrane of epithelial cells, SIgA are transported into the lumen by transcytosis after the apical cleavage of pIgR. The cleaved pIgR, known as secretory component, is capable of protecting SIgA from digestion by numerous bacterial proteases at mucosa. It is estimated that about three to five grams of SIgA are secreted in the intestinal lumen each day which can account for its primary role in the regulation of mucosal immune systems.
In addition to secreted forms, IgA exists in a membrane-anchoring form (mIgA) which is encoded by joining a membrane exon after the CH3 exon by RNA splicing. The membrane exon is translated into a membrane-anchoring peptide which corresponds to three different environmental segments, an extracellular peptide, referred to as the mIg isotype-specific (migis-α) segment consisting of 26-32 amino acid residues, a transmembrane region, and a cytosolic tail, respectively. The migis-α segment is varied in sequences and lengths among five Ig isotypes while the transmembrane domain of them is highly conserved. Therefore, the migis-α segment can be served as an antigenic site for targeting mIgA and mIgA-expressing B cells. The resultant antibodies therefore can be used for treating relevant diseases, which can benefit from the elimination of mIgA-expressing cells or the reduction of IgA antibodies in the immune system, such as IgA lymphoctyes, IgA nephropathy (IgAN), Henoch-Schönlein purpura (HSP), Celiac disease, etc.
In IgA1, not IgA2, two splicing acceptor sites are present in the membrane exon and isoforms of mα1 mRNAs are generated by alternative joining of the donor in the CH3 exon towards either acceptor in the membrane exon. Two resultant isoforms differ in that the long isoform (mα1L) has extra six amino acid residues at the N-terminal of migis peptide which is 26 a. a. residues in length for the short isoform (mα1S). The expression amount of mα1L is about twice more than that of mα1S in mIgA1-expressing B cells. In studies, two mα1 alleles can produce the long and the short isoforms of migis-α, and three mα2 alleles produce the short form exclusively.
While migis-α has been proposed as an antigenic site for preparing antibodies that can bind to mIgA and cause the lysis of mIgA-expressing B cells since as early as 1990 (U.S. Pat. No. 5,079,334), no such antibodies have ever been prepared. In our previous paper [Hung et al., Mol. Immunol. 48(15-16): 1975-1982 (2011)], a number of mAbs that bind strongly to synthetic migis-α polypeptides and to migis-α-containing recombinant proteins in ELISA were prepared. However, among those mAbs, only one, denoted as 29C11, had a marginally detectable binding to mIgA on B cell lines, such as IgA1-expressing DAKIKI cells or Daudi cells transfected with mα1 chains. There were no data provided concerning whether 29C11 can induce lysis of mIgA-expressing B cells by apoptosis, ADCC, or other mechanisms.